1. Field of the Invention
The present invention relates to a negative electrode for a lithium secondary battery, a lithium secondary battery, and manufacturing methods thereof.
2. Description of the Related Art
In recent years, with the advance of environmental technology, development of power generation devices (e.g., solar power generation devices) which pose less burden on the environment than conventional power generation methods has been actively conducted. Concurrently with the development of power generation technology, development of power storage devices such as lithium secondary batteries, lithium-ion capacitors, and air cells has also been underway.
In particular, demand for lithium secondary batteries have rapidly grown with the development of the semiconductor industry, as in the cases of electrical appliances such as mobile phones, smartphones, portable information terminals such as laptop computers, portable music players, digital cameras, medical equipment, and next-generation clean energy vehicles such as hybrid electric vehicles (HEV), electric vehicles (EV), and plug-in hybrid electric vehicles (PHEV), and the lithium secondary batteries are essential for today's information society as a chargeable energy supply source. Especially in the case of applications for electric vehicles or home electrical appliances such as refrigerators, batteries with higher capacity and higher output are desirable.
A negative electrode used in such a lithium secondary battery (hereinafter “negative electrode for a lithium secondary battery”) is manufactured in such a manner that a layer containing an active material (hereinafter “active material layer”) is formed on one surface of a current collector. Graphite (black lead) which is capable of intercalation and deintercalation of ions serving as carriers (hereinafter “carrier ions”) is a conventional material used as a negative electrode active material. In other words, a negative electrode has been manufactured in such a manner that graphite which is a negative electrode active material, carbon black as a conductive additive, and a resin as a binder are mixed to form slurry, the slurry is applied over a current collector, and the current collector is dried.
In contrast, in the case of using silicon or silicon doped with phosphorus as a negative electrode active material, carrier ions about four times as much as those in the case of using carbon can be inserted, and the theoretical capacity of a silicon negative electrode is 4200 mAh/g, which is significantly higher than a theoretical capacity of carbon (black lead) negative electrode of 372 mAh/g. Thus, silicon is an optimal material for increasing capacity of a power storage device, and lithium secondary batteries using silicon as a negative electrode active material have been actively developed today in order to increase the capacity.
However, as the number of carrier ions to be inserted increases, a change in the volume of an active material in accordance with insertion and extraction of carrier ions in charge-discharge cycles increases, resulting in lower adhesion between a current collector and silicon and deterioration of battery characteristics due to charge and discharge. Further, in some cases, a serious problem is caused in that silicon is deformed and broken to be separated or pulverized, so that a function as a battery cannot be maintained.
In Patent Document 1, for example, as a negative electrode active material, a layer formed using microcrystalline silicon or amorphous silicon is formed in a columnar shape or in a powder form over a current collector formed using a copper foil or the like with a rough surface, and a layer formed using a carbon material such as black lead which has lower electric conductivity than silicon is provided over the layer formed using silicon. This makes it possible to collect current through the layer formed using a carbon material such as black lead even if the layer formed using silicon is separated; thus, deterioration of battery characteristics is reduced.